The present invention relates generally to strain reliefs and more particularly to an inverted strain relief for electrical cables.
Strain reliefs for coaxial electrical cables are generally made of a thermo-plastic material, such as SANTOPRENE®. FIG. 1 illustrates a strain relief 10 formed around a coaxial cable 12 that coupled to a measurement probe 14. The strain relief 10 encircles the cable 12 and tapers from the connection of the coaxial cable 12 to the measurement probe 14 or other type of a device, such as a probe control unit (formally known as a termination box) or the like, to a point 16 along the length of the coaxial cable 12. The strain relief 10 has a pattern of lateral slots 18 formed therein that allows the cable 12 to bend or flex. The length of the strain relief 10 is a function of the coaxial cable diameter and its susceptibility to damage from excessive bending. The ratio of the strain relief length over the coaxial cable diameter increases as the coaxial cable diameter decreases. This results in a smaller diameter coaxial cable generally requiring a longer strain relief. For example, a 0.184 inch diameter coaxial cable 12 has a strain relief length of approximately 1.25 inches.
As the bandwidth of measurement instruments, such as oscilloscopes and the like, increases, there is a corresponding need to produce measurement probe having equal or greater bandwidths. A major difficulty in designing very wide bandwidth measurement probes having bandwidths of 5 GHz and greater is the effects of capacitance and inductance of the probing tip or tips. One solution to this problem is to separate the probing tips from the active circuitry in the probing head of the measurement probe. U.S. Pat. No. 6,704,670 describes a wideband active probing system where the probing tip or tips of the probe are separable from a probe amplifier unit. One or more probe cables are connected to a probe tip unit and selectively connected to the probe amplifier unit for conveying signals received by a probe unit. The probe cables are typically connected to the probe amplifier unit by probe cable connectors which may include strain relief devices. Various types of probe tip units may be connected to the probe amplifier unit. The probe tip unit may contain circuitry ranging from conductor traces to various resistive, capacitive, and/or other electronic elements. An advantage of such a probe design is that it allows the placement of the substantially smaller probe tip unit difficult to reach contacts on a device under test instead of a larger measurement probe containing probe amplifier circuitry.
One requirement for such a probe design is to minimize the length of the probe cables. The longer the probe cable length, the lower the overall bandwidth of the measurement probe. Preferably, the overall cable length of such cables in this type of probing application should be in the range of two inches for an 8 GHZ measurement probe. This in turn, limits the type of strain relief that can be put on the cable. Using a 0.070 inch diameter cable with a standard stain relief as shown in FIG. 1 would require the cable to be longer than the optimum two inch cable length. What is needed is a new type of strain relief for measurement probing applications that does not require a long coaxial cable strain relief.
U.S. Pat. No. 6,489,568 describes a wire protection grommet for installation in a wire pull opening extending between a preset and the cell of a cellular raceway. The grommet includes a central opening extending between the preset and the raceway cell. The central opening has a proximal end that is flared outwardly, as is the inner surface of the central opening. The central opening tapers down to opposing parallel surfaces at the distal end, which is disposed in wire pull opening in the preset. The flared construction gives the proximal end of the central opening a smooth radiused surface that functions as a wire pull strength relief.
U.S. Pat. No. 6,051,795 describes a grommet that is positionable in a panel hole to prevent a fire from spreading from one panel side to the other. The grommet has a inner peripheral surface defining a hollow central opening with a radius less than that of a cable received in the opening. The inner peripheral surface is outwardly radiused in opposing directions from the central opening. This allows the cable to be shifted from a concentric relationship in the hole to a non-concentric relationship.